[LuoguP2999] [USACO10NOV] Chocolate Milk Chocolate Milk

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Consider a point of each path through, it can reach each of the zero point (end point), and each of the read zero (origin) can reach it,

Each node record topological sort can reach a number of zero degree and the reverse side can reach pronounced as the number of zero, it is determined whether the total number is equal to

#include<iostream>
#include<cstring>
#include<cstdio>
using namespace std;

const int MAXN=100010;
const int MAXM=400010;

int n;
int Head[MAXN],num,du[MAXN],cnt1,f1[MAXN];
int _Head[MAXN],_num,_du[MAXN],cnt2,f2[MAXN];

struct Edge{
    int to,nxt;
}e[MAXN],_e[MAXN];

inline void add(int x,int y){
    e[++num].to=y;
    e[num].nxt=Head[x];
    Head[x]=num;
}
inline void _add(int x,int y){
    _e[++_num].to=y;
    _e[_num].nxt=_Head[x];
    _Head[x]=_num;
}

int dfs1(int x){
    if(f1[x]) return f1[x];
    int sum=0;
    if(du[x]==0) ++sum;
    for(int i=Head[x];i;i=e[i].nxt){
        int to=e[i].to;
        sum+=dfs1(to);
    }
    return f1[x]=sum;
}

int dfs2(int x){
    if(f2[x]) return f2[x];
    int sum=0;
    if(_du[x]==0) ++sum;
    for(int i=_Head[x];i;i=_e[i].nxt){
        int to=_e[i].to;
        sum+=dfs2(to);
    }
    return f2[x]=sum;
}

int main()
{
    scanf("%d",&n);
    int x,y;
    for(int i=1;i<n;++i){
        scanf("%d%d",&x,&y);
        add(x,y); ++du[x];
        _add(y,x); ++_du[y];
    }
    for(int i=1;i<=n;++i){
        if(du[i]==0) ++cnt1;
        if(_du[i]==0) ++cnt2;
    }
    for(int i=1;i<=n;++i)
        if(!f1[i]) dfs1(i);
    for(int i=1;i<=n;++i)
        if(!f2[i]) dfs2(i);
    for(int i=1;i<=n;++i)
        if(_du[i]!=0&&f1[i]==cnt1&&f2[i]==cnt2)
            printf("%d\n",i);
    return 0;
}